Steel cars face a weighty decision

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Pressure to improve automotive gas milage is driving efforts to find new ways to reduce the weight of steel car-body structures.

The automobile has been on an extended weight-loss program for several decades now. Ever since the oil shortages of the 1970s and the subsequent adoption of tougher environmental regulations, auto engineers have been whittling away at vehicle weight as a way to boost fuel economy and reduce exhaust emissions. The lighter they can make a vehicle, the lighter its body structure and suspension system could be, and the lighter the engine that would be needed.

This color simulation of the stress distribution in a portion of the ULSAB auto-body structure indicates "hot spots"--regions of high stress--where reinforcement may be required

Knowing that smaller is lighter, design engineers initially responded by downsizing vehicles. Around the same time, traditional full-frame/rear-drive auto architectures began to give way to lower-mass-unibody/front-drive car designs.
When these weight-saving strategies started to reach their limits, attention turned to incorporating more low-density materials such as plastics and aluminum in steel cars. Car designers first applied this materials-substitution approach to interior components such as instrument panels, which are now almost universally made of plastic. Engine blocks are increasingly likely to be fabricated from aluminum rather than heavy cast iron, particularly in higher-performance cars. As a result, today's average car has double the amount of nonferrous materials than it did in 1975.
In the meantime, iron and steel declined from about three-quarters of a car's total mass to about two-thirds. Most of that ferrous fraction is mild steel, a highly formable, low-carbon steel grade with the relatively low yield strength of 20,000 to 23,000 pounds per square inch. But part of the decreasing use of iron and steel can also be attributed to wider application of higher-strength steels, which can often perform the same structural function with less metal. High-strength steel (HSS) grades--iron alloys with yield strengths above 40,000 pounds per square inch--now comprise about one-fifth of the total ferrous content of new cars, and their use is growing steadily.
As a result of the past 30 years of weight-reduction efforts, the typical passenger car dropped about 1,000 pounds, while average gas mileage climbed from 14 to 28 miles per gallon.
The job is nowhere near completed, of course. "Federal requirements for fuel economy and emissions won't be easing up in the future. They can only get tougher," said Glyn Davies, senior staff technical specialist at Ford Motor Co.'s research laboratories in Dearborn, Mich. "The same goes for competition in fuel economy." So automotive engineers are still searching for new ways to shave even more mass from cars. To that end, metallurgical engineers are working to develop or refine higher-performance steels and to ensure that the physical properties of specific steel grades remain consistent from batch to batch. Meanwhile, manufacturing engineers are investigating economical and effective steel-processing methods, and auto-structure engineers are exploring new high-efficiency structural concepts.


The ULSAB body in white, shown as a surfaced concept model, may achieve a weight reduction of 25 percent over conventional configurations

Today's unibody or unitized-body auto structure is a spot-welded assembly of stamped-sheet steel panels and folded channel beams. This stiff-shell structure--what the auto industry calls a body in white (BIW)--typically accounts for one-third of a vehicle's total weight. Cutting the mass of a car's BIW yields secondary weight savings that roughly double the benefit to fuel consumption.

SHEET STEEL RULES
Despite attempts to find efficient alternatives, sheet steel is still the material of choice for car-body structures today and for the foreseeable future, according to auto engineers. "The main reason we use sheet steel for car bodies is that it's the most cost-effective material for the job," said Vinay Shah, senior specialist in body-materials engineering at Chrysler Corp. in Auburn Hills, Mich. The low cost of sheet steel--approximately 33 cents per pound for mildest grades, compared with about $1.50 per pound for aluminum and several times more for polymer composites--is going to be tough to beat, particularly in the near term, Shah noted. Even the use of higher-strength steel grades adds a premium of only 10 to 15 percent to the cost of rolled automotive sheet products, according to a widely accepted rule of thumb.
"Besides being the lowest-cost structural material, we have a tremendous amount of manufacturing experience with steel," said James R. Fekete, a development engineer for General Motors Corp. in Detroit. Steel is also highly formable, especially in sheet form, he added, and its strength, durability, and crashworthiness have been demonstrated over time. Add the auto industry's huge investment in the existing infrastructure for shaping, assembling, and recycling steel, and it is hard to see how the situation will change anytime soon. "Unless there is a big upset in the industry--severe regulatory pressure or some significant innovation--steel cars are going to be around for a long time," Fekete said.
"If you want to reduce mass when designing with steel, there really is only one thing to do--use less," he added. "It sounds simple, but it is really quite profound." In practice, this approach means changing to a higher-strength grade but using it in thinner gauge.
Higher-strength steels come in a spectrum of strength levels. Medium-strength low-alloy (MSLA) steels, which have yield strengths from 25,000 and 40,000 pounds per square inch, are also called dent-resistant steels because they were first used in exposed body panels. Now widely applied in new car designs, MSLA steels have the same carbon levels as mild steels, but are solution-strengthened by dissolving more phosphorus or manganese alloy ingredients into the melt during manufacturing. These additions make MSLA somewhat more costly than milder grades.
High-strength low-alloy (HSLA) steel varieties, with yield strengths in the range of 40,000 to 75,000 pounds per square inch, are precipitation-hardened by the addition of small amounts of titanium or niobium, which produces fine dispersions of carbide particles. Though HSLA has been produced for decades, it has found use in cars only in the last 10 to 15 years. In some new models, however, 60 percent of the total steel content is composed of MSLA and HSLA grades. HSLA steel is somewhat less formable than the lower-strength grades, a drawback that is the subject of vigorous research by metallurgists.
Farther up the strength ladder are dual-phase steels, which feature yield strengths from 75,000 to 150,000 pounds per square inch. Dual-phase steels, typically mostly ferrite and some martensite (two iron-alloy compositions), are produced by alloying iron with magnesium and silicon, followed by special processing. Dual-phase steels have relatively high formability, Fekete said. "The downside is there is a lot of alloy in these grades, making them more costly and more difficult to weld and zinc-coat for corrosion protection." To date, dual-phase steels have found only limited commercial application.
Steel grades featuring even higher-strength levels (115,000 to 215,000 pounds per square inch), such as the fully martensitic varieties, are available, but their limited formability slows commercial use, according to Fekete. "The hardness and brittleness of the martensite phase means you have to use special processing methods such as roll forming, tube milling, or even hot stamping."

STRENGTH OR STIFFNESS?
Underlying these decisions regarding material selection is the overall design goal for the particular body structure component. "You're designing either for strength or for stiffness," Fekete said. "That's what's driving the thickness of the material." The thickness of steel car parts is usually determined by the degree of required stiffness, but in about 20 percent of the applications the important property is strength. "If you're designing for stiffness, an increase in strength level won't give you anything because the elastic modulus of the material doesn't change," Fekete said.
"When you design for strength, you're primarily trying to handle crash loads," said Bruce Emmons, president of Autokinetics, an engineering services firm in Rochester Hills, Mich. "Suspension loads are also a concern, but they tend to be much lower magnitude than crash loads," he said. "Generally, we want to use higher-strength steels in crash-sensitive parts such as impact beams, bumper bars, rockers, and B-pillar reinforcements. More recently, however, the focus has moved toward greater stiffness, which is desired for improved ride, vibration, and harshness [NVH] quality." Higher stiffness targets are something relatively new in American car design, he said, joking that structural stiffness levels used to be set just high enough "to allow the car doors to close."

Laser welding permits the welding of joints accessible on only one side. These continuous joint connections increase structural rigidity compared with spot welding

"If you want to make a standard steel unibody stiffer, you add mass," Emmons said. "As a designer, the idea is to try to add stiffness faster than you add mass, but there's a limit. Since today's cars use a very mature design architecture, it's getting harder and harder to squeeze out improvements."

HIGH-STRENGTH STEEL APPLICATIONS
A good example of current lightweighting practice in the auto industry was the use of high-strength steel by Chrysler engineers in the underbodies of the 1995 JA cars--the Chrysler Cirrus and Dodge Stratus sedans. "We were determined to give the Cirrus/Stratus the best performance, in terms of vehicle dynamics, of any production vehicle from Chrysler up until that time, and high-strength steel gave us the structural performance, combined with weight reduction,"said John Siekirk, the company's program administrator for stamping.
"The decision to try and use high-strength steels wherever possible was made very early in the design process, and was based totally on what they could bring to the car in terms of performance,"said Jim Cotton, senior engineer for body-chassis structure design. "We were working with a platform that was a little longer than the vehicle it replaced and that was targeted for a major weight reduction."At the same time, the performance targets specified significant improvements in structure, major reductions in NVH, improved durability performance, and budget restraints. "In terms of strength, its ability to reduce weight, and cost, high-strength steel by far provided the best solution to these multiple challenges,"Cotton said.
In total, about 100 body parts out of 200 body-in-white parts were made of high-strength steel. Chrysler engineers were able to move up to higher strengths and reduce gauge thicknesses compared with the initial product designs on many parts. "We started out with about 40 parts in 40,000 pounds per square inch, and then moved up to 50,000 pounds per square inch with many of them, thus enabling us to move down in gauge without sacrificing performance,"Siekirk said. As a result, Chrysler was typically able to reduce the gauges of the underbody parts by 33 percent compared with the gauges required if the components were made of mild steel. The gauge reductions led to weight savings of about one-third.
Once the optimal gauge and materials were selected, forming challenges still remained. "We had to work very hard at controlling springback with the thicker structural members,"said Anthony Vogel, program administrator for advanced stamping and assembly engineering. "High-strength steel seems to have a memory of its own, and it likes to go back there. Sometimes to get the angle you require, it's hard to hit it directly to compensate for springback."
The Chrysler team tried to form the HSS parts in form dies rather than in conventional draw dies whenever possible. "In cases where we needed to use a draw die--in forming the rails, for instance--Chrysler die-process engineers incorporated pressure rings and modified the radii,"Vogel said. Design engineers also softened some of the harsher angles of the components, making them less severe.
Using HSS also necessitated higher press tonnages than expected. "We very often found that we had to use more tonnage than expected to make the parts. We needed to calculate the tonnage required for the entire panel using a thicker gauge,"Vogel said. "As an example, we calculated that 300 tons of pressure would be required to form one part, yet when we went into tryout we found we needed about 500 tons to make an acceptable part."The disparity was attributed to insufficient dynamic stress-strain data on the HSS material as well as the lack of certain forming considerations in the manufacturing analysis.
Chrysler's lightweighting efforts are tied directly to the design demands of individual new vehicle platform projects, according to Shah. "Each platform team is trying to solve its own problems,"he said. "For example, we're seeing greater use of higher-strength steels in heavy-duty vehicles such as Jeeps and light trucks, in structural parts such as frames, rails, floor pans, reinforcements, and beams."
Chrysler engineers are also planning to use more aluminum. The 1998 LH cars, for example, will have some aluminum body panels. However, Shah said the company will use a lot more high-strength steel and aluminum once it develops greater experience--particularly manufacturing experience--with these materials. "We figure that in the next five years, we'll be able to produce an overall weight reduction of 10 to 15 percent."

TUBE HYDROFORMING
Another aspect of the current weight-savings effort is a trend toward making efficient structural parts using tube-hydroforming processing. Tube hydroforming is a procedure used to manufacture complex shapes in tubular components that can serve as alternatives to closed-section stamped assemblies. The process leads to parts consolidation, eliminates the need for weld flanges, and reduces weight.
A typical tubular-hydroforming process starts with a straight, round tube of seam-welded sheet steel, according to Douglas Viohl, commercial director of Vari-Form in Warren, Mich., which is currently hydroforming about 3 million auto parts a year. The tube is then bent to the shape of the die in a computer-numerical-control bender and placed in a forming die cavity where about 5,000 to 10,000 pounds per square inch of water pressure is injected into the tube. This presses the sheet material against the die surfaces, making the metal conform to the shape of the cavity.
"It's kind of like plastic blow molding, except we're using high-pressure water,"Viohl said. A single tool can produce hundreds of thousands of parts. The process is suitable for forming mild steel, HSLA steels, and aluminum alloys. Viohl noted that the Vari-Form tube- hydroforming process uses pressure sequencing--forming is accomplished in separate low-pressure and high-pressure stages to improve part-detail quality.
Several new vehicles incorporate tube-hydroformed parts. Probably the most notable recent application of the technique is the HSS subframe of the new Chevrolet Corvette, which is manufactured using a proprietary GM tube-hydroforming process. Elsewhere, a hydroformed instrument panel beam in new Chrysler minivans replaces the previous 17-piece stamped assembly, saving approximately 12 pounds. The current-model Dodge Dakota features a U-shaped radiator enclosure that is hydroformed.
Engineers said that hydroformed tubing is also useful for fabricating engine cradles, lower and upper longitudinal body rails, steering-column energy-absorption bellows, D-pillars for station wagons, and various body cross members.
Viohl forecasts that hydroformed tubing will be in every vehicle in five to 10 years. "The key to success is designing specifically for hydroforming,"he said.

TAILOR-WELDED BLANKS
Another emerging technology in U.S. car manufacturing that has potential for mass reduction, cost cutting, and performance improvement is tailor-welded blanks. Tailor-welded blanks are sheet-metal patchworks made of steels with different gauge thicknesses, strengths, or coatings. A single blank of steel is fabricated by welding two or more separate sheets together before it is stamped or formed into a specific part. This enables product and manufacturing engineers to "tailor" the blank so each material's best attributes--gauge, strength, and coating--are located precisely where needed. In terms of weight reduction, the process enables car makers to put thickness and strength where needed, and in turn optimize the amount of steel used, according to Darryl Tinay, sales engineer for Shiloh Industries Inc. in Mansfield, Ohio, which does this kind of fabrication work. Developed in Europe, tailor-welded blanks have been used there for many years.
Tinay explained that two basic joining processes are used to make tailored blanks: laser welding, "which gives you a smooth seam, but needs precision edge trimming for a close fit-up," and mash seam welding, a speedier process in which the blanks are overlapped and copper wheels press the metal sheets together as electric current is passed through them. "Mash seam welding is almost like forging; you heat the metal, then press it together," Tinay said.

A computer rendering of the Autokinetics modular frame concept, which has been designed to be sufficiently stiff to allow for the cost-effective use of high-strength stainless-steel-sheet and thin-wall castings

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The Chrysler JA car uses tailor-welded blanks in its engine-compartment rails, where a 2.0-millimeter-gauge, 50,000-pound-per-square-inch steel blank is laser-welded to a 1.0-millimeter-gauge blank of the same metal. The rails were designed to improve structural performance in frontal impacts and reduce cost by improving materials use and reducing tooling costs. The two-piece HSLA "galvannealed" blanks are produced by Utilase Blank Welding Technologies in Detroit.

Tailor-welded blanks are almost becoming a standard procedure, Tinay said. Other car-body applications include body side frames, door inner panels, motor-compartment rails, center-pillar inner panels, and wheelhouse/shock-tower panels.


ULTRALIGHT STEEL AUTO
While steel is the primary structural material for today's cars, the steel industry is working hard to promote its continued use in the future. A prominent steel-industry effort to develop a low-mass steel car is being sponsored by the Ultralight Steel Auto Body (ULSAB) Consortium, a group of 33 major sheet-steel producers from around the world. The ULSAB group recently entered the validation phase of a $22 million project aimed at demonstrating a unibody-type steel structure that is stiffer and cheaper than comparable traditional body structures but weighs about 25 percent less.

ULSAB was established to do precompetitive technology research, according to Darryl C. Martin, Detroit-based director of the Automotive Applications Committee of the American Iron and Steel Institute, an association of North American steel manufacturers. The consortium has hired Porsche Engineering in Troy, Mich., to design the body structure--a relatively straightforward, evolutionary proposal that uses a holistic design approach; high-strength steel alloys (30,000 to 53,500 pounds per square inch); and advanced manufacturing methods including tube hydroforming, tailor-welded blanks, laser welding, and adhesive bonding.

The specific goal is a five-passenger sedan design with a 220-pound BIW that can be economically produced at a rate of 100,000 units per year. ULSAB has set specific targets regarding static torsion and bending characteristics, as well as first BIW harmonics mode. "The idea of holistic design is to look at the entire system as a whole rather than an assembly of individual components--do the finite-element analysis, find the trouble spots and correct them, then go back and look at the entire structure again and make adjustments," Martin explained. "With enough iterations, the hot spots are eliminated, which indicates that the entire structure is sharing the workload."

In the first phase of the project, Porsche engineers created a finite-element-beam model for the initial analysis that allowed them to simulate different beam cross sections and material thicknesses for their rigidity and load-flow characteristics. Based on the specified packaging and the developed beam concept, the connections and dimensions of the beams were then designed to achieve minimum weight. A finite-element-shell model was then built for detailed examination of member cross sections, material thicknesses, and overall design. Porsche predicted that the ULSAB design could achieve a 24-percent mass reduction and save $150, according to Martin.

ULSAB recently unveiled an exterior styling concept created by A&D Concepts Inc. in Farmington Hills, Mich., as part of the second phase of the project. The remainder of phase 2 will include feasibility, detailed design, and engineering analyses to build demonstration hardware, as well as validation manufacturing, assembly, and structural analyses. The phase 3 vehicle-development portion of the project will finish in early 1998 with fully assembled bodies in white. ULSAB and Porsche have selected vendors to produce various sections of the ULSAB demonstration BIWs. Each vendor is responsible for building tools and necessary hardware and for producing finished parts.

Martin noted that ULSAB will soon reveal the results of a similar design effort aimed at light trucks and sport utility vehicles.

Despite the predicted 25-percent weight reduction, Ford's Davies thinks ULSAB's approach to designing lightweight car bodies is unlikely to help automakers reduce the weight of their steel unibodies more than 15 percent. According to Davies, ULSAB's holistic design approach is not one that car makers would be willing or able to apply extensively to their vehicle design work. "It is an approach that is not as appropriate or applicable to the real-life business of designing cars as some people apparently think," he said. Other automotive engineers agreed with that view. As one said, "ULSAB has to do a lot more work to really prove out the feasibility of their approach, which doesn't seem to have much basis in reality."

This less optimistic view is supported by a recent report sponsored by the International Lead-Zinc Research Organization, which predicted that the practical limit for body-structure weight reduction by 2005 is only 11 to 15 percent.


STAINLESS-STEEL CAR
A more radical approach to automotive structural design is being pursued by Autokinetics and Armco Inc., a specialty steel producer in Middletown, Ohio. The concept is for a stainless-steel body structure based on an innovative modular frame architecture developed by Emmons of Autokinetics. The new structure will be practical to manufacture and could reduce mass 40 to 50 percent, Emmons said.

The complex joints of the Autokinetics modular frame will be thin-wall stainless-steelvacuum castings and connect roll-formed or die-stamped stainless-steel-sheet members


"The basic concept of the Autokinetics frame is a modular structure that acts as a stiff box in the front and rear," he explained. "The front and rear boxes are connected by channel sections and the roof, which makes for a great deal of torsional stiffness." Two additional zones in the front and rear serve as crushable sections to protect the center passenger compartment in crashes.

"Our innovation was getting away from the requirement that we design for stiffness," Emmons said. "Our modular frame is significantly stiffer than required, so it can be designed for strength." The frame, he added, "is a kind of space-frame architecture based on triangulated configurations and tension/compression members. It's less beamlike than conventional architectures--more like an aerospace design."

This structural approach enables the use of stainless steel, which "now gets very few applications in cars, mostly in the exhaust system," said Joe Douthett, Armco's market and product manager for the automotive industry. "People know that stainless steel is corrosion-resistant and expensive, but they often aren't aware that it has strength properties superior to HSLA. It also has good ductility, and good repairability as well."

"We expect the stainless-steel frame to be less costly," Emmons said. "Though the base alloy costs are moderate, component costs will be very competitive." The material premium, he explained, is counterbalanced by only having to use about half as much material because of its higher strength and lower scrap rate (about 3 percent). Stamping processes typically have a 40-percent scrap rate due to the formation of engineering scrap or offal--the apron of sheet metal that remains after a part is stamped out.

To make the modules, formed channels of cold-rolled stainless steel are welded to thin-wall cast joints. This approach minimizes the forming operations on high-strength wrought materials, and makes maximum use of the ability of vacuum castings to accommodate complexity.

The modular frame rails and struts are made from an austenitic stainless-steel alloy similar to T304, the metal used for kitchen sinks and flatware. The flat-rolled strip will be provided by Armco in an as-cold-rolled condition to produce yield strengths above 120,000 pounds per square inch yet retain good ductility (20- to 25-percent elongation).

The main structural members are to be produced using contour roll forming, a process in which a series of wheeled dies successively alters the cross section of a component. Like extrusion processes, roll forming yields a part with a continuous profile. The Sawhill Tubular Division in Warren, Ohio, is expected to do the roll forming. Progressive die stamping will be used to make the individual struts, which are axially stiff tension/compression members positioned on the load paths.

The joint material will be thin-wall castings of a specially developed duplex stainless alloy from Armco called Nitronic 19D. The joints will be fabricated using a special sand-casting technique, also called countergravity casting, that is a variation of the Hitchiner process. During casting, the stainless-steel melt is drawn into a porous sand mold with a vacuum. Pulling the melt in with a vacuum allows the process to produce thin walls down to 2 millimeters thick, said Emmons, who added that volume production using the casting technique has not yet been demonstrated. Alloy Engineering and Casting Co. (a part of Digitron) in Champaign, Ill., will do the joint casting work.

The auto assembly will be joined using the familiar spot, laser and MIG welding techniques. "Stainless steel has few joining problems," said Douthett.

The project engineers have completed the initial stage of computer analysis, and have constructed a complete three-eighths-scale plastic model. Autokinetics and Armco are now negotiating to start the prototyping phase of the project. "The project has exceeded our expectations," Douthett said. "We hope to build six to 12 actual frames, maybe by this fall."

Emmons said that the stainless-steel BIW design is sized to satisfy the requirements is the 80-mile-per-gallon "supercar" project being conducted by the Partnership for a New Generation of Vehicles (PNGV), an industry/government research consortium. The PNGV goal is to develop and market a 1,200-pound car with the same performance, convenience, and price as today's cars in the next decade or two.

PNGV researchers are searching for structural materials that will allow them to meet that difficult weight goal. "At this point, the only thing one can say for sure is that no single material or type of material will dominate," said Bill Miller, GM representative on the PNGV materials-technology team. "The cars of the future will continue to be a mix of materials." The real question is how much of each will be included.

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